The present invention relates to a DC motor phase control system. More particularly, the invention relates to a phase control system of a DC motor wherein phase control is continuous even during deceleration of the motor.
The control circuit of a DC motor using a controlled rectifier, such as a thyristor, usually comprises a transformer connected to an AC power supply via its primary winding. The anodes of a first pair of thyristors are connected to the opposite end terminals of the secondary winding of the transformer and produce a positive rectified output. The cathodes of a second pair of thyristors are connected to the opposite end terminals of the secondary winding of the transformer and produce a negative rectified output. A DC motor is connected between the cathodes of the first pair of thyristors and the anodes of the second pair of thyristors in common and an intermediate point of the secondary winding of the transformer.
When the first pair of thyristors of the control circuit are in conductive condition or ON, the motor rotates in a forward direction. When the second pair of thyristors are in conductive conditon or ON, the motor rotates in the reverse direction. Thus, the motor speed is controlled by controlling the timing of the conductive periods of the thyristors. That is, the motor speed is controlled by utilizing a firing angle control circuit to control the firing angles of the thyristors.
Usually, firing phase control is provided by obtaining the synchronous pulse at the zero point of the AC power supply voltage and producing a firing pulse at a timing delayed by a specified period from the synchronous pulse. In this firing phase control method, acceleration, deceleration and constant speed control of the motor is continuously provided. Thus, for example, in acceleration control, the firing angle is gradually reduced, the conductive time of the first pair of thyristors for forward rotation of the motor is hastened, and the armature current gradually increases, whereby the speed of motor increases.
On the other hand, a reverse electromotive force proportional to the rotary velocity of the motor is produced across the winding of the motor and the rotary velocity of the motor reaches the specified level after a specified period of time. The reverse electromotive force then becomes constant, so that the acceleration control is completed. When the known motor control circuit is utilized for deceleration control, a longer deceleration time is required for the following reasons. When an attempt is made to shorten the deceleration time, smooth speed control of the motor is unattainable.
When the firing angle is gradually increased, the timing of the conductive periods of the first pair of thyristors is gradually delayed and the armature current is decreased. This results in motor deceleration. When the firing angle is rapidly increased, however, the speed of the motor, and therefore the reverse electromotive force of the motor winding, cannot follow up such a change and does not change rapidly. As a result, after the reverse electromotive force becomes greater than the power supply voltage, the first pair of thyristors are reversely biased. Thereafter, the first pair of thyristors are not fired and the armature current becomes zero, until the motor stops rotating. For this reason, a longer deceleration time is required.
If it is attempted to shorten the deceleration time by firing the second pair of thyristors with a firing pulse of the timing when the reverse electromotive force becomes larger than the power supply voltage, the armature current suddenly flows in the reverse direction and the rotation of the motor is not smooth.
The known control circuit is insufficient for controlling a driving motor for a machine tool which requires high speed and high precision operation. There is thus a need for a DC motor phase control system which shortens the deceleration time of a DC motor and insures smooth deceleration control.
The principal object of the invention is to provide a DC motor phase control system which eliminates the disadvantages of the known control circuit.
An object of the invention is to provide a DC motor phase control system which shortens the deceleration time of a DC motor.
Another object of the invention is to provide a DC motor phase control system which provides smooth deceleration control for a DC motor.
Still another object of the invention is to provide a DC motor phase control system of simple structure, which is inexpensive in manufacture and functions efficiently, effectively and reliably to provide a DC motor with a shorter deceleration time than known systems and with simultaneous smooth deceleration control.
Yet another object of the invention is to provide a DC motor phase control system in which armature current of the motor is changed continuously and smoothly from positive to zero and to negative during the deceleration time by delaying the pulse indicating the zero point of the AC power supply voltage in accordance with the actual motor speed, and by setting the delayed pulse generating time to the reference time of the firing pulse.
Another object of the invention is to provide a DC motor phase control system which provides smooth deceleration control by continuously changing the armature current of the motor to a negative value from a positive value via zero, and to shorten the deceleration time by using negative current as the control current in deceleration control of the motor.